The present invention relates to a system and method for producing a quadrature amplitude modulated (xe2x80x9cQAMxe2x80x9d) signal. More particularly, the present invention relates to a system and method for dynamically producing a phase-corrected QAM signal as a function of the difference in the phase relationship between the two component signals of the phase-corrected QAM signal and the phase relationship between the two input signals, typically referred to as the xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d signals.
One use for a QAM modulator is in a wireless communication system, although the present inventive system and method is not limited to wireless communication systems. A conventional QAM modulator typically receives an in-phase signal (xe2x80x9cI signalxe2x80x9d), a quadrature signal (xe2x80x9cQ signalxe2x80x9d), each of which contain digital data to be communicated to a receiver, and a first carrier signal. The I and Q signals are typically phase shifted 90xc2x0 each from the other. A first carrier signal is typically used to produce a second carrier signal by means of a phase shifting circuit which shifts the phase of the first carrier signal by 90xc2x0. The I signal is used to modulate the first carrier signal to thereby produce a first component signal and the Q signal is used to modulate the second carrier signal to thereby produce a second component signal. The first and second component signals are conventionally added together to produce an output QAM signal which is then transmitted, either over a wireless or wired system, to a receiver.
The output of the QAM modulator is characteristically a constellation of signal points when viewed in the I-Q plane. The minimum distance (xe2x80x9cdminxe2x80x9d) between any two points of the signal constellation can be seen as a measure of the susceptibility of the communication system to degradation caused by noise. Noise typically causes the data being received by the receiver to be misinterpreted thereby increasing the bit error rate of the system and resulting in the retransmission of the bits received in error. The higher the bit error rate, the more retransmissions required and the less data throughput realized by the communication system. One way to increase the data throughput is to increase the order of modulation. However, as the order of modulation increases, the more points populate the signal constellation resulting in a smaller dmin of the signal constellation and consequently increasing the susceptibility of the communication system to noise.
Ideally, the phase shift between the first and second carrier signals in the modulator is 90xc2x0 which will result in the maximum dmin for a given modulation order and associated signal constellation. In practical systems, however, there is always some phase imbalance, i.e., a phase relationship between the two carrier signals of other than 90xc2x0, which results in a smaller dmin for the signal constellation.
Typical prior art systems attempt to solve the phase imbalance problem a variety of ways. One prior art system and method is to characterize the phase imbalance for a particular communication system and then statically multiply one of the two carrier signals by a constant amount to compensate for the characterized phase imbalance. The problem with this approach is that the phase imbalance may vary depending on non-constant factors, such as ambient temperature. Additionally, these prior art systems are only effect when the phase imbalance is relatively small. Another prior art system and method may attempt to adjust the level of one or both of the input signals to a QAM modulator as a function of some measured parameter, such as ambient temperature, in an open-loop feedback system. Such open loop control systems may provide some limited degree of control and these systems typically increase the complexity and cost of a QAM modulator by requiring a CPU, memory and attendant circuitry. The degree of control attainable is only as good as the program in the CPU.
Yet another prior art system may employ digital techniques to correct the phase imbalance by using intermediate frequency (xe2x80x9cIFxe2x80x9d) based components. These systems suffer from the added complexity and expense of the added IF components such as CPUs, up-converters, multiple filter stages, etc. Yet other typical prior art systems and methods may additionally attempt to compensate for phase imbalance due to a change in ambient temperature by compensating one or both of the carrier signals by a predetermined amount. These systems require the added complexity and cost of temperature measuring equipment along with the attendant signal processing hardware and software.
One embodiment of the present invention avoids the problems and limitations of the prior art by providing a feedback loop to dynamically control the amount of phase shift applied to the carrier signal as a function of the difference in the phase relationship of the two component signals comprising the output QAM signal and the phase relationship of the I and Q input signals. The inventive system provides dynamic, closed loop control based on the current operating conditions while adding only a few simple, inexpensive components.
Accordingly, it is an object of the present invention to obviate many of the above problems and limitations in the prior art and to provide a novel system and method for generating a phase corrected QAM output signal.
It is another object of the present invention to provide a novel system and method for controlling the amount of phase shift applied to the carrier signal as a function of the difference between the phase relationship of the two component signals comprising the output QAM signal and the phase relationship between the input I and Q signals.
It is yet another object of the present invention to provide a novel system and method of producing an error signal as a function of the relative phase imbalances between the input and output signals in a QAM modulator where the error signal is used to phase correct one of the two carrier signals for the QAM modulator.
It is still another object of the present invention to provide a novel system and method for a phase correcting M-QAM modulator operating in the RF frequency range.
It is a further object of the present invention to provide a novel system and method for producing a phase corrected QAM signal representative of an I and a Q input signal where the I signal modulates a first carrier signal and the Q signal modulates a second carrier signal produced by applying the first carrier signal to one or more phase shifting circuits.
It is yet a further object of the present invention to provide a novel M-QAM modulator comprising a feedback loop for dynamically adjusting the phase relationship between the two carrier signals as a function of the difference between the phase relationship of the two carrier signals and the phase relationship of the I and Q input signals.
It is still a further object of the present invention to provide a novel system and method for producing a QAM signal in a phase correcting QAM modulator where one of the carrier signals is derived by applying the other carrier signal to at least one phase shifting circuit where the amount of phase shift applied is a function of the difference in phase between a first signal representative of the phase relationship of the two component signals comprising the QAM output signal and a second signal representative of the phase relationship of the two input signals to the QAM modulator.
It is an additional object of the present invention to provide a novel system and method for providing closed-loop feedback control for dynamically controlling the amount of phase shift applied to a first carrier signal for producing a second carrier signal where the amount of phase shift applied is a function of the difference between the phase relationship of the two component signals comprising an output QAM signal and the phase relationship between the two input (I and Q) signals.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.